Automated external defibrillators (AEDs) are generally able to monitor and analyze electrocardiogram (ECG) data obtained from a patient and determine whether the patient's ECG indicates a "shockable" or "non-shockable" cardiac rhythm (i.e., a cardiac rhythm that may be treated with a defibrillation pulse). Commonly accepted medical protocol recommends treating certain cardiac rhythms, such as ventricular fibrillation (VF) and pulseless ventricular tachycardia (VT), with rapid application of a defibrillation pulse. On the other hand, potentially perfusing cardiac rhythms are generally not treated by application of a defibrillation pulse. These "non-shockable" rhythms include those with QRS complexes being present, such as supraventricular tachycardia and bradycardia. Cardiac asystole (i.e., a lack of cardiac activity) is also considered to be "non-shockable," since a shock provides no benefit during asystole.
An AED typically obtains ECG data from a patient through electrodes placed on the patient. The AED evaluates the ECG data and makes a binary shock/no-shock decision based on the ECG evaluation. The AED then reports the shock/no-shock decision to the operator of the AED. If the AED detects a VF or VT cardiac rhythm in the patient, for example, the AED typically reports "Shock Advised" on a display, charges a defibrillation capacitor inside the defibrillator, and when instructed by the operator of the AED, delivers a defibrillation pulse from the capacitor to the patient. If, on the other hand, the AED detects a non-shockable cardiac rhythm (e.g., asystole or a rhythm with QRS complexes), the AED simply reports "No Shock Advised." As noted, cardiac asystole represents the absence of electrical activity in the heart. Cardiac asystole may be found initially in a patient, it may develop over time during a resuscitation effort, or it may occur for some time after a defibrillation shock. Defibrillation therapy is generally neither effective nor indicated in the treatment of asystole. Chest compressions and artificial respirations (i.e., cardiopulmonary resuscitation, or CPR) may be performed on the patient, but normally are not effective in treating asystole when it is the initial rhythm. As noted by the American Heart Association (AHA) in its Textbook of Advanced Cardiac Life Support (1994), asystole unfortunately "most often represents a confirmation of death rather than a rhythm to be treated."
The AHA encourages medical directors of pre-hospital care to establish criteria for those providing basic life support in the field to determine when to cease providing treatment, including defibrillation, to a patient, particularly in circumstances where a lack of resources and/or risk to rescuers from continuing to treat the patient outweigh the likelihood of successful resuscitation. Such risks include the risk of vehicular accidents during high-speed emergency transport and the risk of withholding basic life support from another patient needing medical assistance in favor of continuing to attempt to resuscitate the patient in an asystolic condition.
A caregiver providing basic life support to a patient may not recognize when the patient's heart is in an asystolic condition. At the present time, AEDs are intended for use by minimally-trained responders and indicate whether delivery of a defibrillation pulse is advised, but do not identify specific ECG rhythms. Without knowing when a patient's heart is in an asystolic condition, a caregiver may continue to apply basic life support techniques, such as defibrillation and CPR, to a patient for a time longer than is medically useful and perhaps to the risk of self and others. In addition, without the ability to recognize cardiac asystole, a caregiver is also not able to perform meaningful triage.